Self-adaptive Positive-sequence Current Quick-break Protection Method for Petal-shaped Power Distribution Network Trunk Line

ABSTRACT

The invention relates to a self-adaptive positive-sequence current quick-break protection method for a petal-shaped power distribution network trunk line. The method comprises the following steps: step 1, calculating a positive-sequence voltage phasor and a positive-sequence current amplitude at a protection installation position when a fault occurs, acquiring and storing a positive sequence impedance value of a protected line; judging a fault type, and judging a fault direction; step 2, when a fault direction element judges that a fault occurs in the forward direction, selecting a self-adaptive current quick-break protection setting formula according to the fault type, and when positive sequence current measured by protection is larger than a protection setting value, judging that the protected line has a short-circuit fault, and making a circuit breaker trip quickly. Compared with the prior art, the method provided by the invention has enough sensitivity and does not change along with the change of the line length and the system operation mode.

CROSS REFERENCE TO RELATED APPLICATION

The invention relates to the field of relay protection of electric powersystem distribution networks, and relates to a single-endedself-adaptive current protection method of a distribution network, inparticular to a self-adaptive positive-sequence current quick-breakprotection method for a petal-shaped power distribution network trunkline.

BACKGROUND OF THE INVENTION

With the improvement of users' requirements for the reliability of thepower distribution network, petal-shaped grid structures are graduallyapplied to the distribution network, and petal-shaped distributionnetwork protection is increasingly important to the safety andreliability of the power system. The petal-shaped urban powerdistribution network is combined to operate, the grid structure isspecial, and the power distribution network trunk line is relativelyshort. While a short-circuit fault occurs, fault current is large, whichseriously threatens the safe and reliable operation of the powerdistribution network system. Therefore, petal-shaped distributionnetwork protection is very important to ensure the reliability andsafety of the modern power distribution network system.

At present, the petal-shaped power distribution network trunk line usesoptical fiber current differential protection as the main protection,and overcurrent protection as the backup protection. However, thecurrent differential protection relies heavily on the synchronization ofthe communication data and the reliability of the communication channelOnce a communication problem occurs, failure or malfunction of opticalfiber differential protection may be caused. Overcurrent protection hasthe problems of high time delay and poor selectivity. The differentfault locations may cause the protection to fail to accurately identifythe fault line and expand the removal range. Therefore, overcurrentprotection is not conducive to ensuring the electric energy supply ofthe load in the petal-shaped power distribution network.

In view of the problems existing in busbar power distribution networkline protection, many scholars focus on self-adaptive current protectionin the research of single power distribution network protection.“Self-adaptive Relay Protection and Its Prospects” proposesself-adaptive current protection that calculates the backside impedanceof the protection, but when the line is short, there is a problem ofinsufficient sensitivity. For the petal-shaped power distributionnetwork, there is no single-ended quantity protection that can actquickly and has sufficient sensitivity. Therefore, in order to ensurethe reliability and safety of the power supply of the petal-shaped powerdistribution network trunk line, it is of great significance to studynew current protection based on single-ended quantity information.

SUMMARY OF THE INVENTION

To solve the problems, the invention discloses a self-adaptivepositive-sequence current quick-break protection method for apetal-shaped power distribution network trunk line. This method is basedon the relationship between the positive sequence voltage and thepositive sequence current at the protection installation position duringa phase-to-phase short-circuit fault and the impedance value of theprotected line, combined with power directional elements to constructsingle-ended current quantity protection criterion for the petal-shapeddistribution network trunk line to quickly identify the short-circuitfault within the protection range. It not only overcomes the problem ofinsufficient sensitivity of the existing current quick-break protection,but also has the advantage of not being affected by changes in a systemoperation mode and a grid structure.

To solve the technical problems, the invention adopts the followingtechnical scheme:

A self-adaptive positive-sequence current quick-break protection methodfor a petal-shaped power distribution network trunk line comprises thefollowing steps:

step 1, calculating a positive-sequence voltage phasor {dot over (U)}₁and a positive-sequence current amplitude I₁ of a protectioninstallation position when a fault occurs based on the obtained voltagepower frequency quantity {dot over (U)}_(apre), {dot over (U)}_(bpre),{dot over (U)}_(cpre) of each phase during normal operation, voltagepower frequency quantity {dot over (U)}_(a), {dot over (U)}_(b), {dotover (U)}_(c) of each phase and current power frequency quantity İ_(a),İ_(b), İ_(c) of each phase when a fault occurs;

obtaining and storing the positive sequence impedance value Z_(L1) ofthe protected line;

utilizing the fault component of each phase current at the protectioninstallation position to determine the fault type; at the same time,utilizing the power directional element adopting a 90° wiring mode todetermine the fault direction based on the obtained fault type;

step 2, selecting a self-adaptive positive sequence quick-break currentprotection setting formula according to the fault type when the faultdirectional element is judged to have a fault in the forward direction,and judging that the protected line has short-circuit fault and making acircuit breaker trip quickly when the protection detects that thepositive sequence current is greater than the protection setting value.

Moreover, the self-adaptive positive sequence quick-break currentprotection setting formula is as follows:

$I_{ZDZ} = {\frac{K_{rel}{\overset{.}{U}}_{P}}{Z_{L1}}}$

where, {dot over (U)}_(P)={dot over (U)}₁ when a three-phaseshort-circuit fault occurs, {dot over (U)}_(P)={dot over (U)}₁−({dotover (U)}_(xpre)/2) when a two-phase phase-to-phase short-circuit faultoccurs, {dot over (U)}_(xpre) is voltage power frequency quantity of thenon-faulty phase X during normal operation, and K_(rel) is a reliabilitycoefficient.

Moreover, the voltage amplitude value is {dot over (U)}_(xpre) duringnormal operation; and a voltage measurement amplitude value 40 ms beforethe fault occurs is taken.

Moreover, the method for judging the fault type is as follows:

when (m|İ_(mgC)|≤|İ_(mgA)|)∩(m|İ_(mgC)|≤|İ_(mgB)|), the fault type is ABtwo-phase short-circuit fault;

when (m|İ_(mgA)|≤|İ_(mgB)|)∩(m|İ_(mgA)|≤|İ_(mgC)|), the fault type is BCtwo-phase short-circuit fault;

when (m|İ_(mgB)|≤|İ_(mgC)|)∩(m|İ_(mgB)|≤|İ_(mgA)|), the fault type is CAtwo-phase short-circuit fault;

when the above conditions are not met, the fault type is ABC three-phaseshort-circuit fault;

where, İ_(mgA), İ_(mgB), İ_(mgC) are separately the fault components ofA-phase, B-phase and C-phase at the protection installation positionrespectively, and m is the setting coefficient.

Moreover, value range of the setting coefficient m is 4˜8.

Moreover, criterion of the power direction is as follows:

${{{- 180}{^\circ}} + \varphi} \leq {\arg\frac{{\overset{.}{U}}_{\phi\phi}}{{\overset{.}{I}}_{\phi}}} \leq \varphi$

where, {dot over (U)}_(ϕϕ) and İ_(ϕ) are separately voltage and currentphasors of the protection installation position respectively, Φ refersto A, B, C; and φ is the line impedance angle.

Moreover, the reliability coefficient K_(rel) is 1.2.

The advantages and beneficial effects of the invention are as follows:

The self-adaptive positive-sequence current quick-break protectionmethod for the petal-shaped power distribution network trunk line isdisclosed based on the relationship between the positive sequencecurrent and the positive sequence voltage at the protection installationposition when the protected line fails in the forward direction.Compared with the existing methods, the method has sufficientsensitivity and does not change with changes in line length and systemoperation mode.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a 10 kV petal-shaped power distributionnetwork.

FIG. 2 is a two-phase phase-to-phase short-circuit fault compositesequence network diagram.

FIG. 3 is an equivalent circuit when three-phase short-circuit faultoccurs

Explanation of labels in the figures is as follows:

In FIG. 1, E_(S) is the equivalent phase potential of the system, Z_(S)is the equivalent internal impedance of the system, A, B, C, D, and Erepresent the bus, f represents the fault point, 1, 2, 3, 4, 5, 6, 7, 8,9, 10 are the serial numbers of the protection, LD1, LD2, LD3, LD4represent the load, and I′_(f) and I″_(f) are the fault currents on bothsides of the fault point.

In FIG. 2, U_(B1) represents the positive sequence voltage at the B bus,U_(f1) represents the positive sequence voltage at the fault point,I′_(f1) and I″_(f1) are the positive sequence currents on both sides ofthe fault point; Z_(m)=Z_(AB)+αZ_(BC), Z_(n)=(1−α)Z_(BC)+Z_(CA).

In FIG. 3, Z_(AB) is the impedance of line AB, Z_(BC) is the impedanceof line BC, Z_(CA) is the impedance of line CDA, α is the ratio of thedistance between the fault point and bus bar B to the total length ofthe BC line, U_(A), U_(B), and U_(C) are phase voltages on busbars A, Band C, and I_(f) is the current of the fault point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be further described in detail below through specificexamples. The following examples are only descriptive and notrestrictive, and the protection scope of the invention cannot be limitedby this.

A self-adaptive positive-sequence current quick-break protection methodfor a petal-shaped power distribution network trunk line utilizes therelationship between the positive sequence current and the positivesequence voltage at the line protection installation position to derivethe positive sequence current quick-break setting value, and comparesthe magnitude of the positive sequence current with the setting value torealize the judgment of short-circuit fault, the specific steps are asfollows:

(1) As shown in FIG. 1, it is a schematic diagram of a 10 kVpetal-shaped power distribution network specifically applied in thisexample. A voltage acquisition device at the protection positionacquires the voltage power frequency quantity {dot over (U)}_(apre),{dot over (U)}_(bpre), {dot over (U)}_(cpre) of each phase during normaloperation and the voltage power frequency quantity {dot over (U)}_(a),{dot over (U)}_(b), {dot over (U)}_(c) of each phase when a faultoccurs, and a current acquisition device acquires current powerfrequency quantity İ_(a), İ_(b), I _(c) of each phase. A data processingdevice calculates the positive sequence voltage phasor {dot over (U)}₁and the positive sequence current amplitude I₁ at the protectioninstallation position while a fault occurs.

The positive sequence impedance value Z_(L1) of the protected line isstored in the protection device in advance.

(2) The fault components of each phase current at the protectioninstallation position are used to distinguish the three-phaseshort-circuit fault from the two-phase short-circuit fault. The powerdirectional element adopting the 90° wiring mode is utilized to judgethe direction of the fault.

(3) The self-adaptive positive sequence quick-break current protectionsetting formula is selected according to the fault type when the faultdirectional element is judged to have a fault in the forward direction,and the protected line is judged to have a short-circuit fault and acircuit breaker is made to trip quickly when the protection detects thatthe positive sequence current is greater than the protection settingvalue. The self-adaptive positive sequence quick-break currentprotection setting formula is as follows:

$I_{ZDZ} = {\frac{K_{rel}{\overset{.}{U}}_{P}}{Z_{L1}}}$

where, {dot over (U)}_(P)={dot over (U)}₁ when a three-phaseshort-circuit fault occurs, {dot over (U)}_(P)={dot over (U)}₁−({dotover (U)}_(xpre)/2) when a two-phase phase-to-phase short-circuit faultoccurs, {dot over (U)}_(xpre) is voltage power frequency quantity of thenon-faulty phase X during normal operation, and K_(rel) is a reliabilitycoefficient.

In the step (1), the voltage phase {dot over (U)}_(pre) during normaloperation can be voltage measuring power frequency quantity 40 ms beforea fault occurs.

In step (2), the fault type data are as shown in table 1 afterzero-sequence current is determined.

TABLE 1 Identification of fault type Fault types Criterions AB two-phaseshort circuit (m | İ_(mgC) |≤| İ_(mgA) |)∩(m | İ_(mgC) |≤| İ_(mgB) |) BCtwo-phase short circuit (m | İ_(mgA) |≤| İ_(mgB) |)∩(m | İ_(mgA) |≤|İ_(mgC) |) CA two-phase short circuit (m | İ_(mgB) |≤| İ_(mgC) |)∩(m |İ_(mgB) |≤| İ_(mgA) |) ABC two-phase short circuit The above conditionsare not met.

In the table, İ_(mgA), İ_(mgB), İ_(mgC) are the fault components ofA-phase, B-phase and C-phase at the protection installation positionrespectively, and m is the setting coefficient with a value of 4˜8.

Power direction criterion is as follows:

${{{- 180}{^\circ}} + \varphi} \leq {\arg\frac{{\overset{.}{U}}_{\phi\phi}}{{\overset{.}{I}}_{\phi}}} \leq \varphi$

where, {dot over (U)}_(ϕϕ) and İ_(ϕ) are separately voltage and currentphasors (Φ refers to A, B, C) at the protection installation positionrespectively, and φ is the impedance angle of the line.

In step (3), the reliability coefficient K_(rel) is 1.2.

The self-adaptive positive-sequence current quick-break protection basedon the relationship between the positive sequence voltage and thepositive sequence current at the protection installation position canidentify the short-circuit fault that occurs on the protected line. Theprinciple is as follows:

when a two-phase short-circuit fault occurs on the protected line, thefault composite sequence network diagram is as shown in FIG. 2. FromFIG. 2, it can be seen that the positive sequence voltage and thepositive sequence current at the protection 3 have the followingrelationship:

$\begin{matrix}{{\overset{.}{I}}_{f1} = \frac{{\overset{.}{U}}_{B1} - \frac{{\overset{.}{E}}_{s}}{2}}{\alpha Z_{BC}}} & (1)\end{matrix}$

In the actual power distribution network, the voltage drop on theimpedance in the system and on the line impedance is ignored, Ė_(S) canbe replaced with voltage {dot over (U)}_(pre) during normal operation ofthe non-faulty phase at the protection installation position.

In the case of a three-phase short-circuit fault, the fault equivalentcircuit is as shown in FIG. 3. From FIG. 3, it can be seen that thepositive sequence voltage and the positive sequence current at theprotection 3 have the following relationship:

$\begin{matrix}{{\overset{.}{I}}_{f1} = \frac{{\overset{.}{U}}_{B1}}{\alpha Z_{BC}}} & (2)\end{matrix}$

formula (1) and formula (2) can be combined to get the self-adaptivepositive sequence current quick-break protection setting value:

$\begin{matrix}{I_{ZDZ} = {\frac{K_{rel}{\overset{.}{U}}_{P}}{Z_{L1}}}} & (3)\end{matrix}$

where, {dot over (U)}_(P)={dot over (U)}₁ when a three-phaseshort-circuit fault occurs, {dot over (U)}_(P)={dot over (U)}₁−({dotover (U)}_(xpre)/2) when a two-phase phase-to-phase short-circuit faultoccurs, {dot over (U)}_(xpre) is voltage power frequency quantity of thenon-faulty phase X during normal operation, and K_(rel) is a reliabilitycoefficient.

From formula (3), the protection range η of quick-break protection canbe obtained:

$\begin{matrix}{\eta = \frac{1}{K_{rel}}} & (4)\end{matrix}$

The protection range η is only related to the reliability coefficientK_(rel), and has nothing to do with the line length and the operationmode of the system. When the reliability coefficient K_(rel) is 1.2, theprotection range of quick-break current protection is η=83.3%.

The power direction criterion can be combined to obtain the operationcriterion of the self-adaptive positive sequence current quick-breakprotection:

$\begin{matrix}\left\{ \begin{matrix}{I_{1} \geq I_{ZDZ}} \\{{{{- 180}{¦{^\circ}}} + \varphi} \leq {\arg\frac{U_{\phi\phi}}{I_{\phi}}} \leq \varphi}\end{matrix} \right. & (5)\end{matrix}$

It can be seen from formula (4) that when a fault occurs within theprotection range of the protected line in the forward direction, thepositive sequence fault current is greater than the setting value.Therefore, the self-adaptive positive sequence current quick-breakprotection can quickly and accurately identify short-circuit faultswithin the protection range.

In this example, PSCAD/EMTDC software is used in the 10 kV petal-shapedpower distribution network system as shown in FIG. 1. The referencecapacity of the system is 100 MVA, the reference voltage is 10.5 kV, andthe internal impedance is 0.23Ω. The unit inductance and resistance ofthe cable line are X1=0.063 Ω/km and R1=0.047 Ω/km; the lengths of linesAB, BC, CD, DE, and EA are 2 km, 2 km, 1 km, 1 km, 2 km respectively. Aload with a rated capacity of 2 MVA and a rated power factor of 0.9 isaccessed to each node.

1) Fault Identification of Self-Adaptive Positive Sequence CurrentQuick-Break Protection

In order to verify the influence of different fault types and faultpositions on the self-adaptive positive sequence current quick-breakprotection, the phase-to-phase short-circuit fault and the three-phaseshort-circuit fault occurred at the BC line α=0.1, 0.4, 0.6, 0.9 aresimulated, and the Operation condition of the protection 3 is as shownin Table 1.

TABLE 1 Operation condition of BC line self-adaptive positive sequencecurrent quick-break protection Three-phase Two-phase phase-to-phaseshort-circuit fault short-circuit fault I_(ZDZ)/ I_(l)/ OperationI_(ZDZ)/ I_(l)/ Operation α Protection kA kA condition kA kA condition0.1 3 1.511 12.480 Operate 0.759 6.308 Operate 0.4 3 5.124 10.649Operate 2.565 5.393 Operate 0.6 3 6.941 9.623 Operate 3.473 4.881Operate 0.9 3 8.965 8.290 Nooperate 4.458 4.214 No operate

It can be seen from Table 1:

Within α≤0.833 and I_(d1)>I_(ZDZ), protection 3 will all operate; withinα>0.833, protection 3 will not operate, and the quick-break protectionat both ends of the line has a definite protection range, which canensure sufficient sensitivity.

2) Influence of Line Length on Protection Range

In order to verify the influence of different line lengths on the rangeof self-adaptive positive-sequence current quick-break protection, thesimulation results of CD line protection are shown in Table 2. In Table2, β is the ratio of the distance between the fault point and bus C tothe total length of the CD line.

TABLE 2 Operation condition of line protection 5 Three-phase Two-phasephase-to-phase short-circuit fault short-circuit fault I_(ZDZ)/ I_(l)/Operation I_(ZDZ)/ I_(l)/ Operation β kA kA condition kA kA condition0.1 0.948 7.689 Operate 0.48 3.834 Operate 0.4 3.443 7.12 Operate 1.7273.55 Operate 0.6 4.89 6.757 Operate 2.451 3.368 Operate 0.9 6.753 6.229No Operate 3.382 3.104 No Operate

It can be seen by combining table 1 with table 2:

The line length does not affect the protection range of theself-adaptive positive sequence current quick-break protection. Even ina short line, the protection still has a certain protection range, whichcan ensure the sensitivity of the self-adaptive positive sequencecurrent quick-break protection.

Although the examples and figures of the invention are disclosed forillustrative purposes, those skilled in the art can understand thatvarious substitutions, changes and modifications are possible withoutdeparting from the spirit and scope of the invention and the appendedclaims. Therefore, the scope of the invention is not limited to thecontent disclosed in the examples and figures.

What is claimed is:
 1. A self-adaptive positive-sequence currentquick-break protection method for a petal-shaped power distributionnetwork trunk line, comprising the following steps: step 1, calculatinga positive-sequence voltage phasor {dot over (U)}₁ and apositive-sequence current amplitude I₁ of a protection installationposition when a fault occurs based on the obtained voltage powerfrequency quantity {dot over (U)}_(apre), {dot over (U)}_(bpre), {dotover (U)}_(cpre) of each phase during normal operation, voltage powerfrequency quantity {dot over (U)}_(a), {dot over (U)}_(b), {dot over(U)}_(c) of each phase and current power frequency quantity İ_(a),İ_(b), İ_(c) of each phase when a fault occurs; obtaining and storingthe positive sequence impedance value Z_(L1) of the protected line; andutilizing the fault component of each phase current at the protectioninstallation position to determine the fault type; at the same time,utilizing the power directional element adopting a 90° wiring mode todetermine the fault direction based on the obtained fault type; step 2,electing a self-adaptive positive sequence quick-break currentprotection setting formula according to the fault type when the faultdirectional element is judged to have a fault in the forward direction,and judging that the protected line has short-circuit fault and making acircuit breaker trip quickly when the protection detects that thepositive sequence current is greater than the protection setting value.2. The self-adaptive positive-sequence current quick-break protectionmethod for the petal-shaped power distribution network trunk lineaccording to claim 1, the self-adaptive positive sequence quick-breakcurrent protection setting formula is as follows:$I_{ZDZ} = {\frac{K_{rel}{\overset{.}{U}}_{P}}{Z_{L1}}}$ wherein, {dotover (U)}_(P)={dot over (U)}₁ when a three-phase short-circuit faultoccurs, {dot over (U)}_(P)={dot over (U)}₁−({dot over (U)}_(xpre)/2)when a two-phase phase-to-phase short-circuit fault occurs, {dot over(U)}_(xpre) is voltage power frequency quantity of the non-faulty phaseX during normal operation, and K_(rel) is a reliability coefficient. 3.The self-adaptive positive-sequence current quick-break protectionmethod for the petal-shaped power distribution network trunk lineaccording to claim 1, wherein the voltage phase {dot over (U)}_(xpre)during normal operation can be voltage measuring power frequencyquantity 40 ms before a fault occurs.
 4. The self-adaptivepositive-sequence current quick-break protection method for thepetal-shaped power distribution network trunk line according to claim 1,wherein when (m|İ_(mgC)|≤|İ_(mgA)|)∩(m|İ_(mgC)|≤|İ_(mgB)|), the faulttype is AB two-phase short-circuit fault; when(m|İ_(mgA)|≤|İ_(mgB)|)∩(m|İ_(mgA)|≤|İ_(mgC)|), the fault type is BCtwo-phase short-circuit fault; when(m|İ_(mgB)|≤|İ_(mgC)|)∩(m|İ_(mgB)|≤|İ_(mgA)|), the fault type is CAtwo-phase short-circuit fault; when the above conditions are not met,the fault type is ABC three-phase short-circuit fault; wherein, İ_(mgA),İ_(mgB), İ_(mgC) are separately the fault components of A-phase, B-phaseand C-phase at the protection installation position respectively, and mis the setting coefficient.
 5. The self-adaptive positive-sequencecurrent quick-break protection method for the petal-shaped powerdistribution network trunk line according to claim 4, wherein the valuerange of the setting coefficient m is 4˜8.
 6. The self-adaptivepositive-sequence current quick-break protection method for thepetal-shaped power distribution network trunk line according to claim 1,wherein the criterion of the power direction is as follows:${{{- 1}80^{\circ}} + \varphi} \leq {\arg\;\frac{{\overset{.}{U}}_{\phi\phi}}{{\overset{.}{I}}_{\phi}}} \leq \varphi$where, {dot over (U)}_(ϕϕ) and İ_(ϕ) are separately voltage and currentphasors of the protection installation position respectively, Φ refersto A, B, C; and φ is the line impedance angle.
 7. The self-adaptivepositive-sequence current quick-break protection method for thepetal-shaped power distribution network trunk line according to claim 2,wherein the reliability coefficient K_(rel) is 1.2.